This invention relates to a system for the storage of sulfur and, more particularly, to a system for storing sulfur for prolonged periods of time. Specifically, the invention is concerned with novel techniques for the storage of commercially produced sulfur in subterranean cavities.
Sulfur is the key raw material in the manufacture of sulfuric acid, the largest commodity chemical in the world in terms of tonnage, and is also used in many other industrial, analytical and medical applications throughout the world. Historically, mined natural sulfur has been the primary source of commercially produced sulfur, although this position has given way in recent years to sulfur recovered from crude oil and natural gas processing. Natural sulfur is mined primarily from underground formations by the Frasch hot water injection process, while considerable lesser amounts are obtained from volcanic rock and other types of ores by traditional mining techniques. Recovered sulfur is obtained largely as a byproduct of crude oil and natural gas production operations by the treatment of gaseous hydrogen sulfide streams in Claus Process plants and the like. Regardless of its source, all mined and recovered sulfur must be properly stored prior to its commercial use.
Conventional sulfur storage methods and facilities often involve the use of steam-heated tanks, where molten sulfur is kept at temperatures usually exceeding 260xc2x0 F., or they may involve the accumulation of solid sulfur storage blocks, also known as xe2x80x9cvatsxe2x80x9d, in contained open areas from which pieces of sulfur may be broken off by mechanical means, crushed and shipped in solid form, or remelted and transported in liquid form.
Molten sulfur storage tanks are expensive to fabricate, operate and maintain. Although often suitable for short-term storage, e.g., less than three months or so, their use is not always economically feasible. The capital cost involved in their fabrication, the expenses associated with correcting corrosion problems, and the cost of the energy required to provide a constant source of steam for keeping the stored sulfur in liquid state do not always make molten sulfur storage tanks best suited for the long-term safekeeping of commercially produced sulfur inventories.
Commercially produced sulfur storage vats (sometimes also referred to as xe2x80x9cblocksxe2x80x9d) are formed by pouring molten run-of-mine or recovered sulfur in contained open areas where the sulfur is allowed to cool and solidify by exposure to ambient conditions. Vats tend to pick up water from rain and atmospheric moisture and form sulfuric acid which, through seepage under the solidified blocks of sulfur and through water runoff, becomes a source of soil and water contamination. In addition, when the sulfur is broken off from the vats to be transported, particulate sulfur is often given off which becomes a source of air contamination. To avoid or minimize releases of particulate sulfur, the sulfur scheduled for transportation is often melted in situ prior to shipping it to the desired locations, and this step adds more capital, operating and maintenance costs to the storage system. Vats also tend to retain sporadic pockets of hot molten sulfur which are not always easy to detect, and which therefore constitute an industrial safety hazard. In some locations, the handling, transportation and/or storage of solid sulfur is prohibited, or so encumbered by regulatory controls as to make them commercially unattractive. Conventional sulfur storage techniques and equipment are described in U.S. Pat. Nos. 4,149,837, 4,151,234, 4,171,200, 4,190,627, 4,595,350, 4,705,432, 5,041,275 and 5,340,383.
In recent years, the inventories of recovered sulfur have increased dramatically worldwide, partly because of the implementation of stricter environmental regulations in practically every country in the world. Industrial plant gases and other sources of sulfur from crude oil and natural gas production operations must be treated to remove their sulfur constituents before releasing them to the atmosphere or otherwise disposing of them. The result is that large inventories of byproduct recovered sulfur continue to be generated which often exceed the current demand for sulfur as a commodity chemical. These inventories must be properly stored, sometimes for long periods of time, i.e., for five or ten years, or even longer, until the market demand calls for their use.
From the foregoing, it is apparent that an important need exists for commercially produced sulfur storage means that are not only capable of safekeeping large industrial tonnages of sulfur for long periods of time, but are also cost effective and environmentally sound. The present invention is directed toward providing such means.
It is an object of the present invention to provide a system for the proper and safe storage of both solid and liquid sulfur. Another object of this invention is to provide a commercially practicable technique for the long-term storage of sulfur at relatively low maintenance and operating costs. A specific object of the invention is to provide a commercially practicable system for the storage of xe2x80x9crecovered sulfurxe2x80x9d, that is, sulfur that has been recovered, or produced, as a byproduct of crude oil and natural gas production operations. A further object of the present invention is to provide an environmentally attractive system for storing commercially produced sulfur, which minimizes the release of sulfur compounds to the atmosphere during storage. Another object of this invention is to provide a system for the storage of commercially produced sulfur at a location close to where the sulfur is mined or recovered, thereby minimizing the costs associated with the transportation and the handling of the sulfur in special sulfur storage tanks or vats. Another object of the invention is to provide an environmentally sound system for the long-term storage of commercially produced sulfur from which the sulfur may be easily and inexpensively reclaimed, when needed, by means of pressurized hot water techniques. A further object of the present invention is to provide an improved system for the conservation of an important natural resource, i.e., sulfur, which system will help prevent, or at least minimize, future shortages of this important natural resource as its sources become gradually depleted. These and other objects of the invention will be apparent to those skilled in the art from the description that follows.
The system of this invention centers around the innovative concept of injecting commercially produced sulfur in a mined subterranean cavity. Commercial sulfur, in elemental state, is produced by mining, or as a byproduct of industrial operations such as crude oil and natural gas production operations. Commercially produced sulfur is often found in molten state as well as in solid state. At ambient pressures and temperatures, elemental sulfur is solid. At ambient pressures and elevated temperatures, that is, at atmospheric pressures and at temperatures higher than about 240xc2x0 F., elemental sulfur is liquid. The system of this invention is particularly suited for the long-term storage of recovered sulfur, which is often produced in liquid form. However, the system may also be used to store mined sulfur, produced in solid form or in molten state, and recovered sulfur produced in solid form. According to the technique of this invention, a mined subterranean cavity is first located or created within a naturally occurring earth formation, and a borehole is provided on the surface of the earth and through the earth formation, which penetrates the subterranean cavity near its top, or at some other convenient location on the cavity. The sulfur to be stored is then injected through the borehole by means of pumping equipment adapted to handle the flow of sulfur, or simply by allowing the sulfur to flow by gravity into the subterranean cavity. The injection of sulfur in this manner is continued until a significant portion of the cavity is filled with sulfur. The sulfur is then stored within the cavity by causing it to settle into the lower portion of the cavity. The stored sulfur is maintained in place by the walls of the cavity. The sulfur can be stored for long periods of time in this fashion and then made available for commercial uses, when needed, by withdrawing as much as necessary by means of hot water injection techniques. The sulfur storage technique of this invention thus avoids the need to provide above ground installations for such storage operations and affords the opportunity to efficiently conserve this valuable natural resource for prolonged periods of time under conditions that have minimum or no impact on the environment.
The mined subterranean cavity prescribed by the system of this invention is a cavity formed by the mechanical mining or by the solution mining of underground mineral deposits such as those found in salt beds and salt domes. The mined subterranean cavity should not contain any fractures, and should be located in formations that exhibit low permeability and low porosity, and little or no movement of oxygen-containing groundwater so as to avoid loss of sulfur from the cavity and contamination of the surrounding areas by sulfur acids which otherwise would be produced from oxidation of the sulfur. The cavity is preferably a solution-mined subterranean cavity created by the solution mining of salt in a naturally occurring salt dome. Such cavities are particularly suitable for use in the storage system of this invention because they tend to be impermeable. The exact depth of the cavity, i.e., the distance from the bottom of the cavity to the surface of the earth, is not critical, but, as explained below, less energy is required to reclaim the sulfur and return it to the surface of the earth if the cavity is near the surface. For example, when reclaiming the sulfur through a borehole by melting with pressurized hot water and airlifting the molten sulfur to the surface, less heat is lost through the borehole and less air pressure is needed to lift the sulfur if the storage cavity is near the surface. Cavities located at depths of less than about 3,000 feet below the surface of the earth are preferred for this reason.
When storing commercially produced liquid sulfur using the system of this invention, the sulfur is injected into the borehole through a set of concentric pipes, disposed within the borehole, through which a heating fluid is also circulated at a rate sufficient to prevent the sulfur from freezing within the borehole. The preferred heating fluid is water that has been pressurized and heated so that its boiling point sufficiently exceeds the melting point of the sulfur so as to keep the sulfur in liquid state and prevent it from freezing within the borehole. Specifically, the temperature of the sulfur while in transit through the borehole is kept above about 250xc2x0 F., and preferably between about 270xc2x0 and 300xc2x0 F. Any fluids present in the cavity prior to the injection in this manner are displaced by the incoming sulfur, which is then deposited within the lower part of the cavity and retained therein in solid or liquid form as explained below. This process is continued until the cavity is substantially filled or a prescribed desired volume level is reached. When the molten sulfur is injected into and through an aqueous fluid that is present in the cavity at a temperature substantially below the melting point of sulfur, sulfur droplets freeze and settle, and are retained as solid particles. When the molten sulfur is injected through an aqueous fluid present in the cavity from an injection point terminating near the bottom of the cavity, the bed of particles accumulating on the bottom of the cavity eventually reaches the discharge end of the injection pipe and forms a pool of molten sulfur. The pool is maintained in the molten state, if desired, by injection of pressurized hot water or hot brine at a temperature above the melting point of sulfur.
Since heat losses to an overlying gas are lower than heat losses to an overlying aqueous fluid, a gas-filled cavity is preferred for the storage of molten sulfur when desiring to form and temporarily maintain a pool of molten sulfur. Thus, for example, in the case of a solution mined cavity, the brine or any other aqueous solution remaining after the formation of the cavity is displaced by a gas, such as nitrogen, carbon dioxide or methane, to form a substantially gas-filled cavity, which is then used for the storage of sulfur in the system of this invention.
Formation and maintenance of a pool of molten sulfur within the storage cavity, whether overlain by a gas or any another fluid, allow the immediate reclamation of a portion of the sulfur, and are useful under conditions that require frequent reclamation of some of the stored sulfur in order to balance short-term demands for sale or for use. For long-term storage, i.e., five to ten years, and longer, the pool is allowed to freeze by discontinuing heat input. The frozen sulfur then remains in place indefinitely within the storage cavity.
Alternatively, the liquid sulfur to be stored may be subjected to granulation, as a preliminary step, to generate solid sulfur prills, which are then fed into the borehole, dry or in slurry form, and injected into the subterranean cavity as stipulated above. Injection of sulfur in this manner is conducted at the normal ambient temperatures existing within the borehole and without the need to provide means for imparting additional heat to the sulfur in order to keep it in molten state as it flows through the borehole. The sulfur prills injected in this fashion displace any fluids that may be present inside the cavity. The prills are deposited therein and remain in place within the cavity indefinitely.
When storing solid sulfur in the system of this invention, the sulfur can be melted and then treated as described above. Otherwise, the sulfur is crushed, or ground, and mixed with water, brine or some other suitable aqueous fluid to generate an aqueous slurry of sulfur which is then injected into the borehole and deposited within the subterranean cavity. Any fluids present in the cavity prior to the injection in this fashion are displaced upwards by the incoming sulfur, which is then deposited within the lower part of the cavity and retained therein. This process is continued until the cavity if substantially filled or a prescribed desired volume level is reached. By contrast with known systems for underground storage of natural gas, petroleum oil, refined petroleum products and other such petrochemicals, which are less dense than brine and hence tend to float on it, the system of this invention utilizes sulfur, which is much denser than brine, to displace the brine upwards while sinking to the bottom of the cavity.
Sulfur inventories injected into mined subterranean cavities by the method and system of this invention are conveniently retrieved, as the need arises, by means of hot water injection techniques whereby the stored sulfur is first melted with an injection of hot water under pressure. The molten sulfur is then lifted to the surface of the earth with the aid of pressurized air, which lowers the density of the molten sulfur. Alternatively, the molten sulfur may be pumped to the surface, e.g., with a submersible pump.
The present invention advances the art of sulfur storage and, in particular, provides an improved system for the long-term safe storage of commercially produced liquid sulfur, as well as commercially produced solid sulfur, with minimal operating and maintenance costs, minimal inventory losses and practically no environmental impact. The invention also provides an improved natural resource conservation system: as sources of high-sulfur-containing hydrocarbons become gradually depleted, future shortages of sulfur will be prevented, or minimized, by the judicious storage of this natural resource in the system of this invention.